Saddle Fit Guide
The video is about the effects of poor training methods but it also shows the skeleton of the horse and where the saddle needs to sit.

click horse to see a video (a little shocking) 

Also, see more painted horses here:  http://www.anatomyinmotion.com/vhorse.htm

When you saddle your horse up, you want to put the saddle onto her back – not on the withers and not on the croup. The shaded area either side of the spine is where you want the weight-bearing parts of the saddle to be in contact with the horse. When the horse moves, obviously every part of it moves, but these areas move less than most. Forward of these areas you are into the shoulder which needs to be free to move, and behind these areas you are into a whole bunch of nerves, and with mares, you are also into the reproductive system. Tail swishing is a good sign that you are too far back.  The maximum length of the saddle should be from the point 2 fingers behind the shoulder to the point of the horses’ last rib.  You should actually measure this distance and use the measurement as a guide when purchasing a saddle.  Your saddle should not be longer than this measurement.

The picture above shows the area beyond which, front and back, you do not want to impede your horse with weight or pressure from your saddle. The front rope is hung over the horse at the point where the back becomes the wither. This is the point where, if you take a profile of your horse, it will give a good indication of the pommel width you require.

Making a profile

1. Take a piece of heavy wire (a clothes hanger will do).
2. Bend the wire over your horses withers at the point 2 fingers behind the shoulder.
3. Place the bent wire on a piece of cardboard and trace the outline onto the cardboard.
4. Cut out the cardboard on the line.

The top piece of card should fit exactly onto your horse as in diagram 1. The bottom piece of card should fit exactly under your pommel as in diagram 2.

You can either send in the cutout part, or take a measurement of the cutout.  To take a measurement of the cutout, start at the top of the center point of the cutout and measure down 3 inches on both sides and place a mark.  Then measure the distance between these 2 marks.  I can determine the pommel size you need based on this information.

The picture on the right and the picture of the Comfort saddle below, shows the proper angle the saddle should fit on your horse. 

 
Notice how the Startrekk Comfort saddle, below, sits properly on the horse.  The pommel is situated above the horse and on top of the saddle.  The pommel on the Startrekk pulls the saddle together and provides structure and stability to the saddle.